Semiconductor Device and Electronic Device

ABSTRACT

To reduce a variation in the electrical characteristics of a transistor. A potential generated by a voltage converter circuit is applied to a back gate of a transistor included in a voltage conversion block. Since the back gate of the transistor is not in a floating state, a current flowing through the back channel can be controlled so as to reduce a variation in the electrical characteristics of the transistor. Further, a transistor with low off-state current is used as the transistor included in the voltage conversion block, whereby storage of the output potential is controlled.

This application is a continuation of copending U.S. application Ser.No. 14/013,517, filed on Aug. 29, 2013 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a voltage convertercircuit. Furthermore, one embodiment of the present invention relates toa semiconductor device. Still further, one embodiment of the presentinvention relates to an electronic device.

2. Description of the Related Art

A power source circuit is used for generating a power source voltage ofa semiconductor device such as a processor.

The power source circuit is provided with a voltage converter circuitsuch as a charge pump.

The voltage converter circuit includes a plurality of voltage conversionblocks composed of, for example, transistors and capacitors, andconverts a voltage by converting an input potential in accordance with aclock signal.

Examples of a transistor in the voltage converter circuit include atransistor using silicon semiconductor for a channel formation region,and a transistor using a metal oxide semiconductor for a channelformation region. For example, Patent Document 1 discloses a voltageconverter circuit in which a metal oxide semiconductor is used for achannel formation region of a transistor.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-171700

SUMMARY OF THE INVENTION

Conventional voltage converter circuits have a problem in that theelectrical characteristics of a transistor vary with a current flowingthrough a back channel of the transistor.

For example, in the case of an n-channel transistor, the thresholdvoltage shifts in a negative direction with an increase in the currentflowing through the back channel. A negative threshold voltage leads toan increase in off-state current. In order to reduce the off-statecurrent of the n-channel transistor, a negative potential needs to becontinuously applied to a gate, which consumes power accordingly.

In the case where a transistor includes a pair of gates with a channelformation region interposed therebetween and a gate on a back-channelside (hereinafter referred to as a back gate) is in a floating state, acurrent flowing through the back channel is likely to vary due to adrain potential. Therefore, a variation in the electricalcharacteristics of the transistor is easily caused.

An object of one embodiment of the present invention is to reduce avariation in the electrical characteristics of a transistor due to acurrent in a back channel. Another object of one embodiment of thepresent invention is to reduce power consumption. Note that in oneembodiment of the present invention, at least one of these objects needsto be achieved.

In one embodiment of the present invention, a potential generated by avoltage converter circuit is applied to a back gate of a transistorincluded in a voltage conversion block. Since the back gate of thetransistor is not in a floating state, a current flowing through theback channel can be controlled so as to reduce a variation in theelectrical characteristics of the transistor.

In the above embodiment of the present invention, a transistor with lowoff-state current may be used as the transistor included in the voltageconversion block. The transistor with low off-state current allows theoutput potential to be held so as to reduce a variation in the outputpotential in the case where, for example, the supply of a clock signalto the voltage converter circuit is stopped. The use of the transistorwith low off-state current also extends the period during which thesupply of a clock signal can be stopped, resulting in a reduction inpower consumption.

One embodiment of the present invention is a voltage converter circuitincluding a first voltage conversion block, a second voltage conversionblock, and an output control transistor. The first voltage conversionblock includes a first conversion control transistor and a firstcapacitor. The second voltage conversion block includes a secondconversion control transistor and a second capacitor. A first potentialis applied to one of a source and a drain of the first conversioncontrol transistor, and a potential of a gate of the first conversioncontrol transistor varies with a first clock signal. One of a pair ofelectrodes of the first capacitor is electrically connected to the otherof the source and the drain of the first conversion control transistor,and a potential of the other of the pair of electrodes varies with thefirst clock signal. One of a source and a drain of the second conversioncontrol transistor is electrically connected to the other of the sourceand the drain of the first conversion control transistor, the other ofthe source and the drain of the second conversion control transistor hasa second potential, and a potential of a gate of the second conversioncontrol transistor varies with a second clock signal. One of a pair ofelectrodes of the second capacitor is electrically connected to theother of the source and the drain of the second conversion controltransistor, and the other of the pair of electrodes varies with thesecond clock signal. One of a source and a drain of the output controltransistor varies with the second potential. A back gate of at least oneof the first and second conversion control transistors is electricallyconnected to the other of the source and the drain of the output controltransistor or the one of the source and the drain of the firstconversion control transistor.

One embodiment of the present invention is a semiconductor deviceincluding a power source circuit provided with a first voltage convertercircuit and a second voltage converter circuit, an oscillator configuredto output a clock signal to the power source circuit, and a CPU coreconfigured to control whether the operation of the oscillator is stoppedor not. The first voltage converter circuit is configured to generate afirst potential which is a negative potential. The second voltageconverter circuit is configured to generate a second potential which isa positive potential. The CPU core includes a register. The registerincludes a first volatile memory circuit configured to store data in aperiod during which a power source voltage is applied to the CPU core,and a second nonvolatile memory circuit configured to store data in aperiod during which supply of the power source voltage to the CPU coreis stopped. The second memory circuit includes a transistor forcontrolling writing and storage of data. The CPU core is furtherconfigured to control whether the first potential or the secondpotential is applied to a back gate of the transistor for controllingwriting and storage of data.

One embodiment of the present invention is an electronic deviceincluding the semiconductor device.

By controlling a potential of a back gate of a transistor, a currentflowing through the back channel can be controlled so as to reduce avariation in the electrical characteristics of the transistor. Inaddition, the potential of the back gate of the transistor can be kepteven in the case where the supply of a clock signal is stopped.Accordingly, the period during which the supply of a clock signal isstopped can be extended, resulting in a reduction in power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are examples of a voltage converter circuit;

FIGS. 2A and 2B are examples of a voltage converter circuit;

FIGS. 3A to 3C are examples of a voltage converter circuit;

FIG. 4 is a graph showing the off-state current of a transistor;

FIGS. 5A to 5C are examples of a method for driving a voltage convertercircuit;

FIGS. 6A and 6B are examples of a voltage converter circuit;

FIGS. 7A and 7B are examples of a voltage converter circuit;

FIG. 8 is an example of a semiconductor device;

FIG. 9 is an example of a power source circuit;

FIG. 10 is an example of a CPU core;

FIGS. 11A and 11B are examples of a register;

FIG. 12 is an example of a structure of a semiconductor device; and

FIGS. 13A to 13F are examples of an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described. Note that it iseasily understood by those skilled in the art that details of theembodiments can be modified in various ways without departing from thespirit and scope of the invention. Accordingly, the present inventionshould not be limited to, for example, the description of theembodiments below.

Note that the contents in different embodiments can be combined with oneanother as appropriate. In addition, the contents of the embodiments canbe replaced with each other as appropriate.

Ordinal numbers such as “first” and “second” are used in order to avoidconfusion among components and the number of components is not limitedby the ordinal numbers.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

Embodiment 1

Described in this embodiment is an example of a voltage convertercircuit which is one embodiment of the present invention.

An example of the voltage converter circuit of this embodiment includesa first voltage conversion block, a second voltage conversion block, andan output control transistor. Note that two or more first voltageconversion blocks and second voltage conversion blocks may be provided.

The first voltage conversion block includes a first conversion controltransistor and a first capacitor, and the second voltage conversionblock includes a second conversion control transistor and a secondcapacitor.

A first potential is applied to one of a source and a drain of the firstconversion control transistor. The potential of a gate of the firstconversion control transistor varies with a first clock signal.

One of a pair of electrodes of the first capacitor is electricallyconnected to the other of the source and the drain of the firstconversion control transistor, and the potential of the other electrodevaries with the first clock signal.

One of a source and a drain of the second conversion control transistoris electrically connected to the other of the source and the drain ofthe first conversion control transistor. The other of the source and thedrain of the second conversion control transistor has a secondpotential, and the potential of a gate of the second conversion controltransistor varies with a second clock signal.

One of a pair of electrodes of the second capacitor is electricallyconnected to the other of the source and the drain of the secondconversion control transistor, and the potential of the other electrodevaries with the second clock signal.

The potential of one of a source and a drain of the output controltransistor varies with the second potential.

The potential of a back gate of at least one of the first and secondconversion control transistors varies with the first potential or thesecond potential. For example, the back gate of at least one of thefirst and second conversion control transistors is electricallyconnected to the other of the source and the drain of the output controltransistor or the one of the source and the drain of the firstconversion control transistor.

Examples of the voltage converter circuit of this embodiment will befurther described with reference to FIGS. 1A and 1B, FIGS. 2A and 2B,FIGS. 3A to 3C, FIG. 4, FIGS. 5A to 5C, FIGS. 6A and 6B, and FIGS. 7Aand 7B.

The voltage converter circuits illustrated in FIGS. 1A and 1B and FIGS.2A and 2B each include voltage conversion blocks 10_2 to 10_N (N is anatural number of 2 or more) and an output control transistor 13. FIGS.1A and 1B and FIGS. 2A and 2B illustrate examples in which N is 4 ormore.

Each of the voltage conversion blocks 10_1 to 10_N has a function ofconverting a voltage by converting a potential to be input (alsoreferred to as an input potential) into another potential.

Note that a voltage refers to a potential difference between two points.However, in general, a difference between the potential of one point anda reference potential (e.g., a ground potential) is merely called apotential or a voltage, and a potential and a voltage are used assynonyms in many cases. Hence, in this specification, a potential may berephrased as a voltage and a voltage may be rephrased as a potentialunless otherwise specified.

A potential converted by a voltage conversion block 10_K (K is a naturalnumber of N−1 or less) is an input potential of a voltage conversionblock 10_K+1.

Each of the voltage conversion blocks 10_1 to 10_N includes a conversioncontrol transistor and a capacitor.

For example, as illustrated in FIGS. 1A and 1B and FIGS. 2A and 2B, avoltage conversion block 10_X (X is a natural number of 1 to N) includesa conversion control transistor 11_X and a capacitor 12_X.

The potential of one of a source and a drain of the conversion controltransistor 11_X is the aforementioned input potential.

For example, in FIGS. 1A and 1B and FIGS. 2A and 2B, the potential ofone of a source and a drain of the conversion control transistor 11_1 isvariable or fixed. The other of the source and the drain of theconversion control transistor 11_1 is electrically connected to one of asource and a drain of the conversion control transistor 112. In otherwords, one of a source and a drain of a conversion control transistor11_K+1 is electrically connected to the other of a source and a drain ofa conversion control transistor 11_K.

A gate of the conversion control transistor 11_X is electricallyconnected to the source or the drain of the conversion controltransistor 11_X.

For example, in FIGS. 1A and 2A, the gate of the conversion controltransistor 11_X is electrically connected to the other of the source andthe drain of the conversion control transistor 11_X. In that case, ineach of the voltage conversion blocks 10_1 to 10_N, a potential isconverted into a lower potential than before.

In FIGS. 1B and 2B, the gate of the conversion control transistor 11_Xis electrically connected to the one of the source and the drain of theconversion control transistor 11_X. In that case, in each of the voltageconversion blocks 10_1 to 10_N, a potential is converted into a higherpotential than before.

In a voltage conversion block 10_M (M is an odd number of 1 to N), thepotential of a gate of a conversion control transistor 11_M varies witha clock signal CLK1, one of a pair of electrodes of a capacitor 12_M iselectrically connected to the other of a source and a drain of theconversion control transistor 11_M, and the potential of the otherelectrode of the capacitor 12_M varies with the clock signal CLK1. Here,the voltage conversion block 10_M corresponds to the first voltageconversion block.

For example, in FIGS. 1A and 1B, the clock signal CLK1 is input to theother electrode of the capacitor 12_1. Further, the other electrode of acapacitor 12 _(—) m (m is an odd number of 3 to N) is electricallyconnected to one of a pair of electrodes of a capacitor 12 _(—) m−2.

In FIGS. 2A and 2B, the clock signal CLK1 is input to the otherelectrode of the capacitor 12_M.

In a voltage conversion block 10_L (L is an even number of 2 to N), thepotential of a gate of a conversion control transistor 11_L varies witha clock signal CLK2, one of a pair of electrodes of a capacitor 12_L iselectrically connected to the other of a source and a drain of theconversion control transistor 11_L, and the potential of the otherelectrode of the capacitor 12_L varies with the clock signal CLK2. Theclock signal CLK2 has a phase opposite to that of the clock signal CLK1.Here, the voltage conversion block 10_L corresponds to the secondvoltage conversion block.

For example, in FIGS. 1A and 1B, the clock signal CLK2 is input to theother electrode of the capacitor 12_2. Further, the other electrode of acapacitor 12 _(—) l (l is an even number of 4 to N) is electricallyconnected to one of a pair of electrodes of a capacitor 12 _(—) l−2.

In FIGS. 2A and 2B, the clock signal CLK2 is input to the otherelectrode of the capacitor 12_L.

Note that the phrase “the potential varies with the signal” is notlimited to the case where “the signal is directly input so that thepotential has the potential of the signal”, but includes the case where,for example, “a transistor is turned on with the signal so that thepotential varies” or “the potential varies when the signal varies due tocapacitive coupling”.

One of a source and a drain of the output control transistor 13 iselectrically connected to the other of a source and a drain of aconversion control transistor 11_N included in the voltage conversionblock 10_N. A gate of the output control transistor 13 is electricallyconnected to the other of the source and the drain of the output controltransistor 13; however, one embodiment of the present invention is notlimited to this structure and a signal may be input to the gate of theoutput control transistor 13, for example.

The output potential of the voltage converter circuit is stored in acapacitor 14. The capacitor 14 may be formed using, for example,parasitic capacitance generated between a wiring for outputting theoutput potential and another wiring. Alternatively, the capacitor 14 maybe formed using a capacitor additionally provided.

In at least one of the voltage conversion blocks 10_1 to 10_N, the backgate of the conversion control transistor (at least one of theconversion control transistors 11_1 to 11_N) is electrically connectedto the other of the source and the drain of the conversion controltransistor included in another voltage conversion block or the other ofthe source and the drain of the output control transistor 13. Withoutlimitation to the above, the back gate of the conversion controltransistor (at least one of the conversion control transistors 11_1 to11_N) may be electrically connected to the one of the source and thedrain of the conversion control transistor included in another voltageconversion block or the one of the source and the drain of the outputcontrol transistor 13.

For example, in FIGS. 1A and 2A, the respective back gates of theconversion control transistors 11_1 to 11_N are electrically connectedto the other of the source and the drain of the output controltransistor 13. Further, the back gate of the output control transistor13 is also electrically connected to the other of the source and thedrain of the output control transistor 13.

In FIGS. 1B and 2B, the respective back gates of the conversion controltransistors 11_1 to 11_N are electrically connected to the one of thesource and the drain of the conversion control transistor 11_1. Further,the back gate of the output control transistor 13 is also electricallyconnected to the one of the source and the drain of the conversioncontrol transistor 11_1.

Without limitation to the above, as illustrated in FIG. 3A for example,the back gate of the conversion control transistor 11_X in FIG. 1A maybe electrically connected to the other of the source and the drain ofthe conversion control transistor 11_X. In this case, the back gate ofthe output control transistor 13 is electrically connected to the otherof the source and the drain of the output control transistor 13.

As illustrated in FIG. 3B, the back gate of the conversion controltransistor 11_X in FIG. 1B may be electrically connected to the one ofthe source and the drain of the conversion control transistor 11_X. Inthis case, the back gate of the output control transistor 13 iselectrically connected to the one of the source and the drain of theoutput control transistor 13.

As illustrated in FIG. 3C, the back gate of the conversion controltransistor 11_X in FIG. 2A may be electrically connected to the other ofthe source and the drain of the conversion control transistor 11_X. Inthis case, the back gate of the output control transistor 13 iselectrically connected to the other of the source and the drain of theoutput control transistor 13. Without limitation to the above, the backgate of the conversion control transistor 11_X in FIG. 2B may beelectrically connected to the one of the source and the drain of theconversion control transistor 11_X.

When a potential is applied to the back gate of the conversion controltransistor, it is possible to mitigate the effect of the drain potentialon the back channel, and thus to control the current flowing through theback channel of the transistor. Further, the threshold voltage of thetransistor can be controlled. Moreover, since the potential applied tothe back gate of the conversion control transistor is generated by thevoltage converter circuit, an external potential does not need to besupplied additionally, which prevents an increase in the number ofwirings.

The conversion control transistor may be a transistor including anelement of Group 14 (e.g., silicon). The conversion control transistormay also be a transistor with low off-state current. The transistor withlow off-state current includes, for example, a substantially i-typechannel formation region including an oxide semiconductor with a widerbandgap than silicon. The transistor including an oxide semiconductorcan be fabricated by the following manner, for example: impurities suchas hydrogen or water are reduced as much as possible and oxygen issupplied so that oxygen vacancies are reduced as much as possible. Atthis time, the amount of hydrogen regarded as a donor impurity in thechannel formation region, which is measured by secondary ion massspectrometry (also referred to as SIMS), is preferably reduced to lowerthan or equal to 1×10¹⁹/cm³, further preferably lower than or equal to1×10¹⁸/cm³.

The transistor containing the oxide semiconductor has low leakagecurrent due to thermal excitation because of its wide bandgap. Further,the effective mass of a hole is large, which is 10 or more, and theheight of the tunnel barrier is high, which is 2.8 eV or higher. Thus,the amount of tunnel current is small. Furthermore, the number ofcarriers in the semiconductor layer is very small; therefore, theoff-state current can be made low. For example, the off-state currentper micrometer of the channel width at 25° C. is lower than or equal to1×10⁻¹⁹ A (100 zA), preferably lower than or equal to 1×10⁻²² A (100yA). It is preferable that the off-state current of the transistor be aslow as possible; the lowest value of the off-state current of thetransistor is estimated to be about 1×10⁻³⁰ A/μm.

The oxide semiconductor can be, for example, an In-based metal oxide, aZn-based metal oxide, an In—Zn-based metal oxide, or an In—Ga—Zn-basedmetal oxide.

It is also possible to use a metal oxide including another metal elementinstead of part or all of Ga in the In—Ga—Zn-based metal oxide. As theanother metal element, a metal element that is capable of combining withmore oxygen atoms than gallium can be used, for example, andspecifically one or more elements of titanium, zirconium, hafnium,germanium, and tin can be used, for instance. Alternatively, as theanother metal element, one or more elements of lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, and lutetium may beused. These metal elements function as a stabilizer. Note that theamount of such a metal element added is determined so that the metaloxide can function as a semiconductor. When a metal element that iscapable of combining with more oxygen atoms than gallium is used andoxygen is supplied to the metal oxide, oxygen vacancies in the metaloxide can be reduced.

As the transistor with low off-state current, a transistor whose channelformation region is formed using an oxide semiconductor containingindium, zinc, and gallium is used and the level of its off-state currentis described here.

For example, FIG. 4 shows an Arrhenius plot of off-state currentestimated from off-state current per micrometer of channel width W of atransistor having a channel width W of 1 m (1000000 μm) and a channellength L of 3 μm when temperature changes to 150° C., 125° C., 85° C.,and 27° C.

In FIG. 4, for example, the off-state current of the transistor at 27°C. is lower than or equal to 1×10⁻²⁵ A. FIG. 4 shows that the transistorincluding a channel formation region formed using an oxide semiconductorcontaining indium, zinc, and gallium has extremely low off-statecurrent.

The structure of the oxide semiconductor layer cable of being used for atransistor will be described below.

An oxide semiconductor layer is classified roughly into a single-crystaloxide semiconductor layer and a non-single-crystal oxide semiconductorlayer. The non-single-crystal oxide semiconductor layer includes any ofan amorphous oxide semiconductor layer, a microcrystalline oxidesemiconductor layer, a polycrystalline oxide semiconductor layer, ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, and thelike.

The amorphous oxide semiconductor layer has disordered atomicarrangement and no crystalline component. A typical example thereof isan oxide semiconductor layer in which no crystal part exists even in amicroscopic region, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor layer includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor layer has a higher degree of atomic order than theamorphous oxide semiconductor layer. Hence, the density of defect statesof the microcrystalline oxide semiconductor layer is lower than that ofthe amorphous oxide semiconductor layer.

The CAAC-OS film is one of oxide semiconductor layers including aplurality of crystal parts, and most of the crystal parts each fitsinside a cube whose one side is less than 100 nm Thus, there is a casewhere a crystal part included in the CAAC-OS film fits a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. The densityof defect states of the CAAC-OS film is lower than that of themicrocrystalline oxide semiconductor layer. The CAAC-OS film isdescribed in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC film observedin a direction substantially perpendicular to the sample surface (planTEM image), metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction perpendicular tothe c-axis, a peak appears frequently when 2θ is around 56°. This peakis derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis(φ scan) is performed under conditions where the sample is rotatedaround a normal vector of a sample surface as an axis (φ axis) with 2θfixed at around 56°. In the case where the sample is a single-crystaloxide semiconductor layer of InGaZnO₄, six peaks appear. The six peaksare derived from crystal planes equivalent to the (110) plane. On theother hand, in the case of a CAAC-OS film, a peak is not clearlyobserved even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depends onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

Note that each of the oxide semiconductor layers may have astacked-layer structure including two or more layers of an amorphousoxide semiconductor layer, a microcrystalline oxide semiconductor layer,and a CAAC-OS layer, for example.

Note that the aforementioned transistor with low off-state current maybe used as the output control transistor 13.

Next, an example of a method for driving the voltage converter circuitof this embodiment, for example, the voltage converter circuitillustrated in FIG. 1A, will be described with reference to FIGS. 5A to5C. Here, 0 V is applied to one of the source and the drain of theconversion control transistor 11_1, for example. Description is made onthe assumption that the conversion control transistors 11_1 to 11_N eachare the aforementioned n-channel transistor with low off-state current.

First, in a period T1, the clock signal CLK1 becomes a high level (H)while the clock signal CLK2 becomes a low level (L) as illustrated inFIG. 5A.

At this time, the conversion control transistor 11_M and the outputcontrol transistor 13 are turned on. When the potential of the other ofthe source and the drain of the conversion control transistor 11_M ischanged to Vd11_M (the potential of the one of the source and the drainof the conversion control transistor 11_M)+Vth11_M (the thresholdvoltage of the conversion control transistor 11_M), the conversioncontrol transistor 11_M is turned off. The conversion control transistor11_L is off at this time.

Then, in a period T2, the clock signal CLK1 becomes the low level (L)while the clock signal CLK2 becomes the high level (H) as illustrated inFIG. 5B.

At this time, the conversion control transistor 11_M and the outputcontrol transistor 13 are turned off. As the clock signal CLK1 ischanged from the high level to the low level, the potential of the otherof the source and the drain of the conversion control transistor 11_M isdecreased to Vd11_M+Vth11_M−VH (the high-level potential of the clocksignal CLK1). Further, the conversion control transistor 11_L is on atthis time. When the potential of the other of the source and the drainof the conversion control transistor 11_L is changed to Vd11_L (thepotential of the one of the source and the drain of the conversioncontrol transistor 11_L)+Vth11_L (the threshold voltage of theconversion control transistor 11_L), the conversion control transistor11_L is turned off. Accordingly, the potential of the other of thesource and the drain of the conversion control transistor 11_L isconverted into a potential lower than the input potential.

Note that contrary to the voltage converter circuit illustrated in FIG.1A, in the voltage converter circuit illustrated in FIG. 1B for example,the potential of the other of the source and the drain of the conversioncontrol transistor 11_L is converted into a potential higher than theinput potential in accordance with the clock signal CLK1 and the clocksignal CLK2.

In a period T_CLKOFF during which the supply of the clock signals CLK1and CLK2 to the voltage converter circuit is stopped, the conversioncontrol transistor 11_M, the conversion control transistor 11_L, and theoutput control transistor 13 are turned off as illustrated in FIG. 5C.In the case where a transistor with low off-state current is used as theconversion control transistors 11 _(—) l to 11_N, the conversion controltransistor 11_M, the conversion control transistor 11_L, and the outputcontrol transistor 13 have low off-state current, and the potentialgenerated by the voltage converter circuit is held for a certain periodaccordingly. Hence, the supply of the clock signals CLK1 and CLK2 to thevoltage converter circuit can be stopped for a longer time, resulting ina reduction in power consumption.

The above is an example of the method for driving the voltage convertercircuit illustrated in FIG. 1A.

Note that the structure of the voltage converter circuit of thisembodiment is not limited to the above.

For example, the one of the source and the drain of the output controltransistor 13 in the aforementioned voltage converter circuit may beelectrically connected to the other of a source and a drain of aconversion control transistor 11_H (H is a natural number of 1 to N−1)included in a voltage conversion block 10_H. At this time, a back gateof the conversion control transistor 11_H included in the voltageconversion block 10_H is electrically connected to the other of a sourceand a drain of a conversion control transistor 11_I (I is a naturalnumber of H+1 to N) included in a voltage conversion block 10_I. As aresult, the potential of the back gate of the conversion controltransistor 11_H can be made lower than that of the other of the sourceand the drain of the conversion control transistor 11_H.

For example, in the case where the aforementioned n-channel transistorwith low off-state current is used as the conversion control transistor11_H, when the potential of the back gate of the conversion controltransistor 11_H is made lower than that of the other of the source andthe drain of the conversion control transistor 11_H, the thresholdvoltage can be made to shift in the positive direction. It is thuspossible to prevent the threshold voltage of the conversion controltransistor 11_H from shifting in the negative direction due todegradation or the like.

For example, FIGS. 6A and 6B each illustrate the case where H is N−2 andI is N: the one of the source and the drain of the output controltransistor 13 is electrically connected to the other of a source and adrain of a conversion control transistor 11_N−2. Further, each of theback gates of the conversion control transistors 11_1 to 11_N iselectrically connected to the other of the source and the drain of theconversion control transistor 11_N. Accordingly, the potential of eachback gate of the conversion control transistors 11_1 to 11_N can be madelower than that of the other of the source and the drain of theconversion control transistor 11N−2.

A transistor 15 and a capacitor 16 may be additionally provided. At thistime, a gate of the transistor 15 is electrically connected to the otherof a source and a drain of a conversion control transistor 11_P (P is anatural number of 1 to N−3) included in a voltage conversion block 10_P.Further, the one of the source and the drain of the output controltransistor 13 is electrically connected to the other of a source and adrain of a conversion control transistor 11_Q (Q is a natural number ofP+1 to N−2) included in a voltage conversion block 10_Q. Moreover, oneof a source and a drain of the transistor 15 is electrically connectedto the other of a source and a drain of a conversion control transistor11_R (R is a natural number of Q+1 to N−1) included in a voltageconversion block 10_R. Still further, one of a pair of electrodes of thecapacitor 16 is electrically connected to the other of the source andthe drain of the transistor 15, and a potential is applied to the otherelectrode. The capacitor 16 preferably has a higher capacitance than thecapacitors 12_1 to 12_N.

The conversion control transistors 11_1 to 11_P each are preferably atransistor (e.g., a transistor including a channel formation region madeof silicon) which has a higher off-state current than the aforementionedtransistor with low off-state current. The conversion controltransistors 11_P+1 to 11_N each are preferably the aforementionedtransistor with low off-state current.

In the above structure, it is assumed that the conversion controltransistors 11_1 to 11_N are n-channel transistors and the transistor 15is a p-channel transistor. In a period during which the clock signalsCLK1 and CLK2 are supplied to the voltage converter circuit, thetransistor 15 is off because a voltage higher than the threshold voltageis applied between the gate and the source. The values of P and R aredetained so that the transistor 15 can be in an off state at this time.A capacitor 12_P and a capacitor 12_R may each have a capacitancedifferent from that of the other capacitors.

In the case where the supply of the clock signals CLK1 and CLK2 to thevoltage converter circuit is stopped in the above structure, thepotential of the other of the source and the drain of the conversioncontrol transistor 11_P gradually increases due to the off-state currentof each of the conversion control transistors 11_1 to 11_P. In thatcase, the transistor 15 is turned on when the voltage between the gateand the source becomes lower than the threshold voltage, and thepotential of the other of the source and the drain of the conversioncontrol transistor 11_R is stored in the capacitor 16.

With the above structure, in a period during which the clock signal issupplied, delay due to the capacitor 16 can be controlled by blockingthe electrical conduction between the capacitor 16 and the conversioncontrol transistor 11_R; and in a period during which the supply of theclock signal is stopped, the capacitor 16 is brought into conductionwith the conversion control transistor 11_R, so that the potential ofthe other of the source and the drain of the conversion controltransistor 11_R can be stored in the capacitor 16 for a longer time.Note that the voltage drop due to the capacitor 16 occurs when thetransistor 15 is turned on; therefore, the potential of the other of thesource and the drain of the conversion control transistor 11_R ispreferably set to be higher than a desired potential by at least thepotential equal to the voltage drop.

For example, a voltage converter circuit illustrated in FIG. 7A has astructure in which the transistor 15 and the capacitor 16 are added tothe voltage converter circuit illustrated in FIG. 6A, and a voltageconverter circuit illustrated in FIG. 7B has a structure in which thetransistor 15 and the capacitor 16 are added to the voltage convertercircuit illustrated in FIG. 6B.

The gate of the transistor 15 is electrically connected to the other ofthe source and the drain of the conversion control transistor 11_2.Further, the one of the source and the drain of the output controltransistor 13 is electrically connected to the other of the source andthe drain of the conversion control transistor 11_N−2. Furthermore, theone of the source and the drain of the transistor 15 is electricallyconnected to the other of a source and a drain of a conversion controltransistor 11_N−1.

With the above structure, in a period during which the clock signal issupplied, delay due to the capacitor 16 can be controlled by blockingthe electrical conduction between the capacitor 16 and the conversioncontrol transistor 11_N−1; and in a period during which the supply ofthe clock signal is stopped, the capacitor 16 is brought into conductionwith the conversion control transistor 11_N−1, so that the potential ofthe other of the source and the drain of the conversion controltransistor 11_N−1 can be stored in the capacitor 16 for a longer time.

As described above with reference to FIGS. 1A and 1B, FIGS. 2A and 2B,FIGS. 3A to 3C, FIG. 4, FIGS. 5A to 5C, FIGS. 6A and 6B, and FIGS. 7Aand 7B, in examples of the voltage converter circuit of this embodiment,a potential generated by the voltage converter circuit is applied to aback gate of a conversion control transistor included in a voltageconversion block. Since the back gate of the transistor is not in afloating state, a current flowing through the back channel can becontrolled so as to reduce a variation in the electrical characteristicsof the transistor.

Further, in examples of the voltage converter circuit of thisembodiment, a transistor with low off-state current is used as atransistor included in a voltage conversion block. The transistor withlow off-state current allows the output potential to be held so as toreduce a variation in the output potential in the case where, forexample, the supply of a clock signal to the voltage converter circuitis stopped. The use of the transistor with low off-state current alsoextends the period during which the supply of a clock signal can bestopped, resulting in a reduction in power consumption.

Embodiment 2

Described in this embodiment is an example of a semiconductor deviceincluding a power source circuit using the voltage converter circuitshown in Embodiment 1.

An example of the semiconductor device of this embodiment will bedescribed with reference to FIG. 8 and FIG. 9.

The semiconductor device illustrated in FIG. 8 includes a CPU core 501,a master controller 502, a power switch 503, an oscillator 504, a powersource circuit 505, and a buffer (also referred to as BUF) 506.

The CPU core 501 is supplied with a power source voltage VDD and acontrol signal from the master controller 502.

The control signal input is, for example, a write control signal CPU_WEwhich is obtained by converting a write control signal CPU_WE0 outputfrom the master controller 502 by a level shifter (also referred to asLS) 512. The control signal is not limited to this, and includes acontrol signal for power supply in the semiconductor device, a controlsignal for driving each circuit block when an instruction is executedbased on a data signal, and the like.

In the CPU core 501, arithmetic processing is executed in accordancewith the control signal from the master controller 502, so that variousoperations are conducted.

The CPU core 501 has a function of, for example, controlling the supplyof a power source voltage VDD_IN to the CPU core 501. The supply of thepower source voltage VDD_IN is controlled in such a manner that, forexample, the power switch 503 is turned on or of by a power controller521.

The CPU core 501 also has a function of controlling whether theoscillator 504 is stopped or not. The oscillator 504 is controlled by,for example, an enable signal EN input from the CPU core 501.

The CPU core 501 also has a function of controlling which of powersource potentials VDD_CP1 and VDD_CP2 generated by the power sourcecircuit 505 is applied to a register 511. For example, the CPU core 501inputs a control signal to a multiplexer (also referred to as MUX) 514which is a selection circuit, thereby controlling which of the powersource potentials VDD_CP1 and VDD_CP2 is supplied.

The CPU core 501 includes the register 511.

The master controller 502 includes a power controller 521 forcontrolling the power switch 503, and a CPU controller 522 forcontrolling the CPU core 501.

The master controller 502 has a function of generating, in accordancewith an instruction signal of the CPU core 501, a control signal for theCPU core 501, a control signal for the power switch 503, a controlsignal for the oscillator 504, and the like.

For example, the power controller 521 has a function of generating acontrol signal PSW_ON and a control signal PSW_OFF which control thepower switch 503.

The CPU controller 522 has a function of generating the write controlsignal CPU_WE0 for controlling writing to the register 511, a controlsignal for controlling arithmetic processing in the CPU core 501, andthe like.

Note that the supply of the power source voltage VDD_IN to the CPUcontroller 522 is controlled by an interrupt signal.

Whether the power switch 503 is turned on or not is controlled by acontrol signal LS_PSWON. The control signal LS_PSWON is obtained byconverting the control signal PSW_ON output from the power controller521 by a level shifter 513. Whether the power switch 503 is turned offor not is controlled by a control signal PSW_OFF output from the powercontroller 521.

The power switch 503 has a function of controlling whether to output thepower source voltage VDD input from the outside. Note that a powersource voltage with a different value may be generated based on thepower source voltage output from the power switch 503, and then suppliedas the power source voltage VDD to the CPU core 501 and the mastercontroller 502.

The oscillator 504 has a function of generating and outputting a clocksignal CLK. Whether the clock signal CLK is generated by the oscillator504 or not is controlled by the CPU controller 522.

The power source circuit 505 has a function of generating the powersource potentials VDD_CP1 and VDD_CP2 in accordance with the clocksignal CLK.

The power source potentials VDD_CP1 and VDD_CP2 generated by the powersource circuit 505 are supplied as a power source potential BG to a backgate of a transistor included in the register 511 through themultiplexer 514. At this time, the CPU core 501 controls which of thepower source potentials VDD_CP1 and VDD_CP2 is supplied from themultiplexer 514.

The buffer 506 has a function of controlling the transmission of signalsbetween the CPU core 501, and a data bus, an address bus, and a controlbus. For example, a data signal is transmitted between the CPU core 501and the data bus, an address signal is transmitted between the CPU core501 and the address bus, and a control signal is transmitted between theCPU core 501 and the control bus.

Further, an example of a structure of the power source circuit 505 willbe described with reference to FIG. 9.

The power source circuit 505 illustrated in FIG. 9 includes a voltageconverter circuit 551, a voltage converter circuit 552, a level shifter553, and a level shifter 554.

The voltage converter circuit 551 is supplied with a power sourcepotential VSS, a clock signal CLK1, and its inverted clock signal CLK1B.The voltage converter circuit 551 has a function of converting the powersource potential VSS in accordance with the clock signal CLK1 and theinverted clock signal CLK1B, thereby generating and outputting a powersource potential VDD_CP1 which is a negative potential.

The voltage converter circuit 551 may be any voltage converter circuitcapable of generating a negative potential (e.g., the voltage convertercircuits illustrated in FIG. 1A, FIG. 2A, FIG. 3A, and FIG. 3C).

The voltage converter circuit 552 is supplied with a power sourcepotential VCP2, a clock signal CLK2, and its inverted clock signalCLK2B. The voltage converter circuit 552 has a function of convertingthe power source potential VCP2 in accordance with the clock signal CLK2and the inverted clock signal CLK2B, thereby generating and outputting apower source potential VDD_CP2 which is a positive potential.

The voltage converter circuit 552 may be any voltage converter circuitcapable of generating a positive potential (e.g., the voltage convertercircuits illustrated in FIG. 1B, FIG. 2B, and FIG. 3B).

The level shifter 553 is supplied with the power source potential VSS, apower source potential VDD_OSC, and a power source potential VCP1, andis also supplied with the clock signal CLK from the oscillator 504. Thepower source potential VDD_OSC is higher than the power source potentialVSS, and the power source potential VCP1 is higher than the power sourcepotential VDD_OSC. In the level shifter 553, the clock signal CLK isconverted into a clock signal whose high level is equal to the powersource potential VDD_OSC, and the clock signal whose high level is equalto the power source potential VDD_OSC is further converted into a clocksignal whose high level is equal to the power source potential VCP1,whereby the clock signal CLK1 is generated. Note that the inverted clocksignal CLK1B is generated by inverting the clock signal CLK1 whose highlevel is equal to the power source potential VCP1 with use of, forexample, an inverter.

The level shifter 554 is supplied with the power source potential VSS,the power source potential VDD_OSC, and a power source potential VCP2,and is also supplied with the clock signal CLK from the oscillator 504.The power source potential VCP2 is higher than the power sourcepotential VDD_OSC. In the level shifter 554, the clock signal CLK isconverted into a clock signal whose high level is equal to the powersource potential VDD_OSC, and the clock signal whose high level is equalto the power source potential VDD_OSC is further converted into a clocksignal whose high level is equal to the power source potential VCP2,whereby the clock signal CLK2 is generated. Note that the inverted clocksignal CLK2B is generated by inverting the clock signal whose high levelis equal to the power source potential VCP2 with use of, for example, aninverter.

The power source potential VDD_CP1 generated by the voltage convertercircuit 551 and the power source potential VDD_CP2 generated by thevoltage converter circuit 552 are input to the multiplexer 514.

The above is an example of the structure of the power source circuit505.

Next, an example of the CPU core 501 will be described with reference toFIG. 10.

The CPU core 501 illustrated in FIG. 10 includes a decoder 614, anarithmetic controller 616, a register set 620, an arithmetic unit 622,and an address buffer 624.

The decoder 614 includes an instruction register and an instructiondecoder. The decoder 614 has a function of decoding input instructiondata and analyzing the instruction.

The arithmetic controller 616 includes a state generation unit and aregister. Further, the state generation unit includes a register. Thestate generation unit generates a signal for determining the state ofthe semiconductor device.

The register set 620 includes a plurality of registers. The plurality ofregisters include registers functioning as a program counter, a generalregister, and an arithmetic register. The register set 620 has afunction of storing data necessary for arithmetic processing.

The arithmetic unit 622 includes an arithmetic logic unit (ALU) 623. Inthe arithmetic unit 622, arithmetic processing of instruction data inputfrom the arithmetic controller 616 is performed using the ALU 623. Notethat the arithmetic unit 622 may also include a register.

The address buffer 624 includes a register. The address buffer 624 has afunction of controlling the transmission of data signals in the registerset 620 in accordance with the address of an address signal.

A write control signal WE and a read control signal RD are input to theCPU core 501. Moreover, 8-bit data is input to the CPU core 501 via abus 640. A CPU control signal is also input to the CPU core 501.

Data of a 16-bit address and a bus control signal are output from theCPU core 501.

The write control signal WE and the read control signal RD are input tothe arithmetic controller 616, the register set 620, and the addressbuffer 624. The 8-bit data is input to the register set 620 and thearithmetic unit 622 via the bus 640. The arithmetic control signal isinput to the arithmetic controller 616. The arithmetic unit 622 executesarithmetic processing in accordance with the arithmetic control signal.

The 16-bit address data is output from the address buffer 624. The buscontrol signal is output from the arithmetic controller 616.

A data signal, an address signal, and an arithmetic control signal canbe input/output to/from each circuit of the CPU core 501 via the bus 640and a bus 641. The bus 640 is, for example, a data bus, an address bus,or a control bus.

Each register provided in the CPU core 501 has a function of storingdata for a certain period of time in data processing.

Further, an example of a structure of the register (the register 511)which can be used in each circuit block will be described with referenceto FIGS. 11A and 11B.

The register 511 illustrated in FIG. 11A includes a volatile memorycircuit 651, a nonvolatile memory circuit 652, and a selector 653.

The volatile memory circuit 651 is supplied with a reset signal RST, aclock signal CLK, and a data signal D. The volatile memory circuit 651has a function of storing data of the data signal D that is input inresponse to the clock signal CLK and outputting the data as a datasignal Q. The reset signal RST, the clock signal CLK, and the datasignal D are input through the CPU controller 522 and the buffer 506,for example.

The nonvolatile memory circuit 652 is supplied with a write controlsignal WE, a read control signal RD, and a data signal.

The nonvolatile memory circuit 652 has a function of storing data of aninput data signal in accordance with the write control signal WE andoutputting the stored data as a data signal in accordance with the readcontrol signal RD.

In the selector 653, the data signal D or the data signal output fromthe nonvolatile memory circuit 652 is selected in accordance with theread control signal RD, and input to the volatile memory circuit 651.

The nonvolatile memory circuit 652 includes a transistor 631 and acapacitor 632.

The transistor 631, which is an n-channel transistor, functions as aselection transistor. One of a source and a drain of the transistor 631is electrically connected to an output terminal of the volatile memorycircuit 651. Further, a back gate of the transistor 631 is electricallyconnected to the multiplexer 514 illustrated in FIG. 9. The transistor631 has a function of controlling the storage of a data signal outputfrom the volatile memory circuit 651 in accordance with the writecontrol signal WE.

As the transistor 631, the transistor with low off-state currentdescribed in Embodiment 1 can be used.

One of a pair of electrodes of the capacitor 632 is electricallyconnected to the other of the source and the drain of the transistor631, and the other of the pair of electrodes is supplied with the powersource potential VSS. The capacitor 632 has a function of holding chargebased on data of a stored data signal. Since the off-state current ofthe transistor 631 is extremely low, the charge in the capacitor 632 isheld and thus the data is stored even when the supply of the powersource voltage is stopped.

The transistor 633 is a p-channel transistor. The power source potentialVDD is supplied to one of a source and a drain of the transistor 633,and the read control signal RD is input to a gate of the transistor 633.

The transistor 634 is an n-channel transistor. One of a source and adrain of the transistor 634 is electrically connected to the other ofthe source and the drain of the transistor 633, and the read controlsignal RD is input to a gate of the transistor 634.

The transistor 635 is an n-channel transistor. One of a source and adrain of the transistor 635 is electrically connected to the other ofthe source and the drain of the transistor 634, and the power sourcepotential VSS is input to the other of the source and the drain of thetransistor 635.

An input terminal of an inverter 636 is electrically connected to theother of the source and the drain of the transistor 633. An outputterminal of the inverter 636 is electrically connected to the inputterminal of the selector 653.

One of a pair of electrodes of a capacitor 637 is electrically connectedto the input terminal of the inverter 636, and the other of the pair ofelectrodes is supplied with the power source potential VSS. Thecapacitor 637 has a function of holding charge based on data of a datasignal input to the inverter 636.

Note that without limitation to the above, the nonvolatile memorycircuit 652 may include a phase-change random access memory (PRAM), aresistive random access memory (ReRAM), a magnetic random access memory(MRAM), or the like. For the MRAM, a magnetic tunnel junction element(MTJ element) can be used for example.

Next, an example of a method for driving the register 511 illustrated inFIG. 11A will be described.

First, in a normal operation period, the register 511 is supplied withthe power source voltage, the reset signal RST, and the clock signalCLK. At this time, the selector 653 outputs data of the data signal D tothe volatile memory circuit 651. The volatile memory circuit 651 storesthe data of the data signal D that is input in accordance with the clocksignal CLK. At this time, in response to the read control signal RD, thetransistor 633 is turned on while the transistor 634 is turned off.

Then, in a backup period provided immediately before the supply of thepower source voltage is stopped, in accordance with the pulse of thewrite control signal WE, the transistor 631 is turned on, the data ofthe data signal D is stored in the nonvolatile memory circuit 652, andthe transistor 631 is turned off. After that, the supply of the clocksignal CLK to the register is stopped, and then, the supply of the resetsignal RST to the register is stopped. Note that when the transistor 631is on, the back gate of the transistor 631 is supplied with the powersource potential VDD_CP2 which is a positive potential from themultiplexer 514. At this time, in response to the read control signalRD, the transistor 633 is turned on while the transistor 634 is turnedoff.

Next, in a power stop period, the supply of the power source voltage tothe register 511 is stopped. During this period, the stored data is heldin the nonvolatile memory circuit 652 because the off-state current ofthe transistor 631 is low. Note that the supply of the power sourcevoltage may be stopped by supplying the ground potential GND instead ofthe power source potential VDD. Note that when the transistor 631 isoff, the back gate of the transistor 631 is supplied with the powersource potential VDD_CP1 which is a negative potential from themultiplexer 514, so that the transistor 631 is kept off.

Then, in a recovery period immediately before a normal operation period,the supply of the power source voltage to the register 511 is restarted;then, the supply of the clock signal CLK is restarted, and after that,the supply of the reset signal RST is restarted. At this time, beforethe supply of the clock signal CLK is restarted, the wiring which is tobe supplied with the clock signal CLK is set to the power sourcepotential VDD. Moreover, in accordance with the pulse of the readcontrol signal RD, the transistor 633 is turned off, the transistor 634is turned on, and the data signal stored in the nonvolatile memorycircuit 652 is output to the selector 653. The selector 653 outputs thedata signal to the volatile memory circuit 651 in accordance with thepulse of the read control signal RD. Thus, the volatile memory circuit651 can be returned to a state just before the power stop period.

Then, in a normal operation period, normal operation of the volatilememory circuit 651 is performed again.

The above is an example of the method for driving the register 511illustrated in FIG. 11A.

Note that the structure of the register 511 is not limited to thatillustrated in FIG. 11A.

For example, the register 511 illustrated in FIG. 11B has a structure inwhich the transistors 633 and 634, the inverter 636, and the capacitor637 are removed from the register 511 illustrated in FIG. 11A and aselector 654 is added to the register 511 illustrated in FIG. 11A. Forthe same components as those in the register 511 illustrated in FIG.11A, the description of the register 511 in FIG. 11A is referred to asappropriate.

One of the source and the drain of the transistor 635 is electricallyconnected to the input terminal of the selector 653.

In the selector 654, the power source potential VSS to be data or thedata signal output from the volatile memory circuit 651 is selected inaccordance with the write control signal WE2, and input to thenonvolatile memory circuit 652.

Next, an example of a method for driving the register 511 illustrated inFIG. 11B will be described.

First, in a normal operation period, the register 511 is supplied withthe power source voltage, the reset signal RST, and the clock signalCLK. At this time, the selector 653 outputs data of the data signal D tothe volatile memory circuit 651. The volatile memory circuit 651 storesthe data of the data signal D that is input in accordance with the clocksignal CLK. In addition, the selector 654 outputs the power sourcepotential VSS to the nonvolatile memory circuit 652 in accordance withthe write control signal WE2. In the nonvolatile memory circuit 652, thetransistor 631 is turned on in response to the pulse of the writecontrol signal WE, and the power source potential VSS is stored as datain the nonvolatile memory circuit 652.

Then, in a backup period provided immediately before the supply of thepower source voltage is stopped, the selector 654 does not supply thepower source potential VSS but provides electrical conduction betweenthe output terminal of the volatile memory circuit 651 and one of thesource and the drain of the transistor 631 in accordance with the writecontrol signal WE2. Further, in accordance with the pulse of the writecontrol signal WE, the transistor 631 is turned on, the data of the datasignal D is stored in the nonvolatile memory circuit 652, and thetransistor 631 is turned off. At this time, the data of the nonvolatilememory circuit 652 is rewritten only when the potential of the datasignal D is equal to the power source potential VDD. Furthermore, thesupply of the clock signal CLK to the register is stopped, and then, thesupply of the reset signal RST to the register 511 is stopped. Note thatwhen the transistor 631 is on, the back gate of the transistor 631 issupplied with the power source potential VDD_CP2 which is a positivepotential from the multiplexer 514.

Next, in a power stop period, the supply of the power source voltage tothe register 511 is stopped. During this period, the stored data is heldin the nonvolatile memory circuit 652 because the off-state current ofthe transistor 631 is low. Note that the supply of the power sourcevoltage may be stopped by supplying the ground potential GND instead ofthe power source potential VDD. Note that when the transistor 631 isoff, the back gate of the transistor 631 is supplied with the powersource potential VDD_CP1 which is a negative potential from themultiplexer 514, so that the transistor 631 is kept off.

Then, in a recovery period immediately before a normal operation period,the supply of the power source voltage to the register 511 is restarted;then, the supply of the clock signal CLK is restarted, and after that,the supply of the reset signal RST is restarted. At this time, beforethe supply of the clock signal CLK is restarted, the wiring which is tobe supplied with the clock signal CLK is set to the power sourcepotential VDD. In accordance with the pulse of the read control signalRD, the selector 653 outputs to the volatile memory circuit 651 the datasignal corresponding to the data stored in the nonvolatile memorycircuit 652. Thus, the volatile memory circuit 651 can be returned to astate just before the power stop period.

Then, in a normal operation period, normal operation of the volatilememory circuit 651 is performed again.

The above is an example of the method for driving the register 511illustrated in FIG. 11B.

By using the structure illustrated in FIG. 11B, the data of the powersource potential VSS does not need to be written in the backup period,resulting in an increase in operation speed.

Next, an example of a structure of the semiconductor device in thisembodiment will be shown in FIG. 12.

In the semiconductor device illustrated in FIG. 12, a transistor 802including an oxide semiconductor in a channel formation region isstacked over a transistor 801 including silicon in a channel formationregion, and a plurality of wiring layers are stacked between thetransistor 801 and the transistor 802.

The transistor 801 is provided in a semiconductor substrate having anembedded insulating layer. The transistor 801 corresponds to thetransistor 635 illustrated in FIGS. 11A and 11B, for example. Further,the conversion control transistor may have the same structure as thetransistor 801.

The transistor 802 includes a conductive layer 821 a embedded in aninsulating layer 820, an insulating layer 822 over the conductive layer821 a, a semiconductor layer 823 overlapping with the conductive layer821 a with the insulating layer 822 interposed therebetween, conductivelayers 824 a and 824 b electrically connected to the semiconductor layer823, an insulating layer 825 over the semiconductor layer 823 and theconductive layers 824 a and 824 b, a conductive layer 826 overlappingwith the semiconductor layer 823 with the insulating layer 825interposed therebetween, and an insulating layer 827 over the conductivelayer 826.

In that case, the conductive layer 821 a functions as a back gateelectrode of the transistor 802. The insulating layer 822 functions as agate insulating layer of the transistor 802. The semiconductor layer 823functions as a channel formation layer of the transistor 802. Theconductive layers 824 a and 824 b function as a source electrode or adrain electrode of the transistor 802. The insulating layer 825functions as a gate insulating layer of the transistor 802. Theconductive layer 826 functions as a gate electrode of the transistor802. The transistor 802 corresponds to the transistor 631 illustrated inFIGS. 11A and 11B, for example. Note that the conversion controltransistor may have the same structure as the transistor 802.

Through an opening provided in the insulating layer 822, the conductivelayer 824 a is electrically connected to a conductive layer 821 b whichis formed using the same conductive layer as the conductive layer 821 a.The conductive layer 821 b is electrically connected to a gate electrodeof the transistor 801 through a wiring layer 812 embedded in aninsulating layer 811, a wiring layer 813 over the wiring layer 812, anda wiring layer 815 which is over the wiring layer 813 and embedded in aninsulating layer 814. In this case, part of the insulating layer 820 isremoved by chemical mechanical polishing (also referred to as CMP), forexample, so that the surfaces of the conductive layers 821 a and 821 bare exposed.

Furthermore, a wiring layer 830 and a wiring layer 833 are stacked inorder over the transistor 802. The wiring layer 830 is electricallyconnected to the conductive layer 824 b through a wiring layer 829 whichis embedded in the insulating layer 825, the insulating layer 827, andan insulating layer 828 over the insulating layer 827. The wiring layer833 is electrically connected to the wiring layer 830 through a wiringlayer 832 which is embedded in an insulating layer 831 over the wiringlayer 830.

Each component will be further described. Note that each layer may havea layered structure.

Each of the wiring layers 812, 813, 815, 829, 830, 832, and 833 can be,for example, a layer containing a metal material such as molybdenum,titanium, chromium, tantalum, magnesium, silver, tungsten, aluminum,copper, neodymium, ruthenium, or scandium.

Each of the insulating layers 811, 814, 820, 822, 825, and 827 can be,for example, a layer containing a material such as silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, orhafnium oxide.

Note that in order that the semiconductor layer 823 is supersaturatedwith oxygen, insulating layers in contact with the oxide semiconductorlayer 823 (e.g., the insulating layers 822 and 825) each preferablyinclude a layer containing excess oxygen.

The insulating layer containing excess oxygen is formed using a siliconoxide film or a silicon oxynitride film containing a large amount ofoxygen as a result of film formation by a plasma CVD method or asputtering method under the conditions which are set as appropriate.Alternatively, oxygen may be added to at least one of the semiconductorlayer 823 and the insulating layers in contact with the semiconductorlayer 823 by an ion implantation method, an ion doping method, or plasmatreatment.

In addition, blocking layers against oxygen, hydrogen, or water arepreferably provided as the insulating layers 822 and 827 outside theinsulating layers containing excess oxygen. Accordingly, it is possibleto prevent diffusion of oxygen contained in the oxide semiconductorlayer to the outside and entry of hydrogen, water, or the like into theoxide semiconductor layer from the outside. The blocking layers can be,for example, layers containing a material such as silicon nitride,aluminum oxide, aluminum nitride, gallium oxide, gallium oxynitride,yttrium oxide, yttrium oxynitride, aluminum oxynitride, aluminum nitrideoxide, or hafnium oxide.

In the case where the semiconductor layer 823 is surrounded by theinsulating layers containing excess oxygen or the blocking layers, theoxide semiconductor layer can contain oxygen in a proportion which issubstantially the same as that in the stoichiometric composition, or ina supersaturated state in which oxygen exceeding the stoichiometriccomposition is contained.

For example, the insulating layer 822 may be formed by a stack of asilicon nitride layer and a silicon oxynitride layer.

For example, the insulating layer 825 may be formed using a siliconoxynitride layer.

For example, the insulating layer 827 may be formed by a stack of asilicon nitride layer and a silicon oxynitride layer.

For example, the insulating layers 814 and 820 may be formed using asilicon oxide layer which is formed by a CVD method using a tetraethylorthosilicate (TEOS) gas. In that case, the planarity of the insulatinglayers 814 and 820 can be improved.

As the semiconductor layer 823, for example, an oxide semiconductorlayer can be used.

As the oxide semiconductor, for example, any of the oxide semiconductorsshown in Embodiment 1 can be used.

Further, the semiconductor layer 823 may have a layered structureincluding a first oxide semiconductor layer with an atomic ratio ofIn:Ga:Zn=1:1:1, a second oxide semiconductor layer with an atomic ratioof In:Ga:Zn=3:1:2, and a third oxide semiconductor layer with an atomicratio of In:Ga:Zn=1:1:1. By employing this layered structure for thesemiconductor layer 823, the transistor 802 can have a buried channelstructure in which a channel is formed apart from the insulating layers(822 and 825) in contact with the semiconductor layer 823, whereby thetransistor 802 has good electrical characteristics with less variation.

The aforementioned transistor including the oxide semiconductor can befabricated by the following manner impurities such as hydrogen or waterare reduced as much as possible and oxygen is supplied so that oxygenvacancies are reduced as much as possible. At this time, the amount ofhydrogen regarded as a donor impurity in the channel formation region,which is measured by secondary ion mass spectrometry (also referred toas SIMS), is preferably reduced to lower than or equal to 1×10¹⁹/cm³,further preferably lower than or equal to 1×10¹⁸/cm³.

For example, a layer containing oxygen is used as the layer in contactwith the oxide semiconductor layer, and heat treatment is performed;thus, the oxide semiconductor layer can be highly purified.

In addition, the oxide semiconductor layer just after its formation ispreferably supersaturated with oxygen so that the proportion of oxygenis higher than that in the stoichiometric composition. For example, inthe case of using sputtering, the oxide semiconductor layer ispreferably formed under the condition where the proportion of oxygen ina deposition gas is high, and particularly in an oxygen atmosphere(e.g., oxygen gas: 100%).

In a sputtering apparatus, the amount of moisture remaining in adeposition chamber is preferably small. Therefore, an entrapment vacuumpump is preferably used in the sputtering apparatus. Further, a coldtrap may be used.

For formation of the oxide semiconductor layer, heat treatment ispreferably performed. The temperature of the heat treatment ispreferably higher than or equal to 150° C. and lower than the strainpoint of the substrate, more preferably higher than or equal to 300° C.and lower than or equal to 450° C. Note that the heat treatment may beperformed more than once.

A rapid thermal annealing (RTA) apparatus such as a gas rapid thermalannealing (GRTA) apparatus or a lamp rapid thermal annealing (LRTA)apparatus can be used as a heat treatment apparatus for the heattreatment. Alternatively, another heat treatment apparatus such as anelectric furnace may be used.

After the heat treatment, a high-purity oxygen gas, a high-purity N₂Ogas, or ultra-dry air (having a dew point of −40° C. or lower,preferably −60° C. or lower) is preferably introduced into the furnacewhere the heat treatment has been performed while the heatingtemperature is being maintained or being decreased. In that case, it ispreferable that the oxygen gas or the N₂O gas do not contain water,hydrogen, and the like. The purity of the oxygen gas or the N₂O gaswhich is introduced into the heat treatment apparatus is preferably 6Nor higher, more preferably 7N or higher. That is, the impurityconcentration of the oxygen gas or the N₂O gas is preferably 1 ppm orlower, more preferably 0.1 ppm or lower. Through this step, oxygen issupplied to the oxide semiconductor layer, and defects due to oxygenvacancies in the oxide semiconductor layer can be reduced. Note that theintroduction of the high-purity oxygen gas, the high-purity N₂O gas, orthe ultra-dry air may be performed at the time of the above heattreatment.

Note that the oxide semiconductor may be a CAAC-OS.

For example, the oxide semiconductor layer that is a CAAC-OS can beformed by a sputtering method. In that case, the sputtering is performedusing a polycrystalline oxide semiconductor sputtering target. When ionscollide with the sputtering target, a crystal region included in thesputtering target may be separated from the target along an a-b plane;in other words, a sputtered particle having a plane parallel to an a-bplane (flat-plate-like sputtered particle or pellet-like sputteredparticle) may flake off from the sputtering target. In that case, thesputtered particle reaches a substrate while maintaining its crystalstate, whereby a crystal state of the sputtering target is transferredto a substrate. In this manner, the CAAC-OS is formed.

For the deposition of the CAAC-OS, the following conditions arepreferably used.

For example, when the CAAC-OS is formed while the amount of impurityentering the CAAC-OS is reduced, the crystal state of the oxidesemiconductor can be prevented from being broken by the impurities. Forexample, it is preferable to reduce the concentration of impurities(e.g., hydrogen, water, carbon dioxide, and nitrogen) existing in adeposition chamber of a sputtering apparatus. Further, the concentrationof impurities in a deposition gas is preferably reduced. For example, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is preferably used as a deposition gas.

It is preferable that the temperature of the substrate at the time ofdeposition be high. In the case of high substrate temperature, when theflat-plate-like sputtered particle reaches the substrate, migration ofthe sputtered particle occurs, so that a flat plane of the sputteredparticle can be attached to the substrate. For example, an oxidesemiconductor film is deposited at a substrate heating temperature of100° C. to 740° C., preferably 200° C. to 500° C., whereby the oxidesemiconductor layer is formed.

Further, it is preferable to reduce plasma damage at the time ofdeposition by increasing the oxygen percentage in the deposition gas andoptimizing electric power. For example, the oxygen percentage in thedeposition gas is preferably 30 vol % or higher, still preferably 100vol %.

Further, the concentration of impurities such as silicon in the oxidesemiconductor is preferably reduced to lower than 2.5×10²¹ atoms/cm³,preferably less than 4.0×10¹⁹ atoms/cm³, and more preferably less than2.0×10¹⁸ atoms/cm³. Such a reduction in the concentration of impuritiesprevents the CAAC-OS from being damaged by the impurities. Note that asthe impurities, titanium, hafnium, or the like can also be given inaddition to silicon.

Each of the conductive layers 821 a, 821 b, and 826 can be, for example,a layer containing a metal material such as molybdenum, titanium,chromium, tantalum, magnesium, silver, tungsten, aluminum, copper,neodymium, ruthenium, or scandium.

Each of the conductive layers 824 a and 824 b can be, for example, alayer containing a metal material such as molybdenum, titanium,chromium, tantalum, magnesium, silver, tungsten, aluminum, copper,neodymium, ruthenium, or scandium. As the conductive layers 824 a and824 b, a layer of a metal oxide that functions as a conductor andtransmits light can also be used, for example. For example, indiumoxide-zinc oxide or indium tin oxide can be used.

The insulating layers 828 and 831 each can be, for example, a layercontaining an organic resin material.

As illustrated in FIG. 12, an example of the semiconductor device inthis embodiment includes a stack of different transistors, which resultsin a reduction in the circuit area.

The above is the description of an example of the structure of thesemiconductor device illustrated in FIG. 12.

As described with reference to FIG. 8, FIG. 9, FIG. 10, FIGS. 11A and11B, and FIG. 12, in an example of the semiconductor device of thisembodiment, the power source circuit is formed using the voltageconverter circuit shown in Embodiment 1, and the semiconductor device isformed using the oscillator and the CPU core including the register. Inaddition, the CPU core controls whether a positive potential or anegative potential is applied to a back gate of a transistor in theregister. As a result, a variation in the electrical characteristics(e.g., threshold voltage) of the transistor in the register can bereduced.

Embodiment 3

In this embodiment, examples of electronic devices including thesemiconductor device which is one embodiment of the present inventionwill be described with reference to FIGS. 13A to 13F.

An electronic device illustrated in FIG. 13A is an example of a portableinformation terminal.

The electronic device illustrated in FIG. 13A includes a housing 1011, apanel 1012 provided on the housing 1011, a button 1013, and a speaker1014.

The housing 1011 may be provided with a connection terminal forconnecting the electronic device to an external device and a button foroperating the electronic device.

The panel 1012 is a display panel (display). The panel 1012 preferablyhas a function of a touch panel.

The button 1013 is provided on the housing 1011. When the button 1013 isa power button, for example, pressing the button 1013 can turn on or offthe electronic device.

The speaker 1014 is provided on the housing 1011. The speaker 1014outputs sound.

Note that the housing 1011 may be provided with a microphone, in whichcase the electronic device in FIG. 13A can function as a telephone set,for example.

In the electronic device illustrated in FIG. 13A, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1011.

The electronic device illustrated in FIG. 13A functions as one or moreof a telephone set, an e-book reader, a personal computer, and a gamemachine, for example.

An electronic device illustrated in FIG. 13B is an example of a foldingdigital assistant.

The electronic device illustrated in FIG. 13B includes a housing 1021 a,a housing 1021 b, a panel 1022 a provided on the housing 1021 a, a panel1022 b provided on the housing 1021 b, a hinge 1023, a button 1024, aconnection terminal 1025, a storage medium inserting portion 1026, and aspeaker 1027.

The housing 1021 a and the housing 1021 b are connected to each other bythe hinge 1023.

The panels 1022 a and 1022 b are display panels (displays). The panels1022 a and 1022 b preferably have a function of a touch panel.

Since the electronic device in FIG. 13B includes the hinge 1023, it canbe folded so that the panels 1022 a and 1022 b face each other.

The button 1024 is provided on the housing 1021 b. Note that the button1024 may be provided on the housing 1021 a. When the button 1024 is apower button, for example, pressing the button 1024 can control thesupply of a power voltage to the electronic device.

The connection terminal 1025 is provided on the housing 1021 a. Notethat the connection terminal 1025 may be provided on the housing 1021 b.Alternatively, a plurality of connection terminals 1025 may be providedon one or both of the housings 1021 a and 1021 b. The connectionterminal 1025 is a terminal for connecting the electronic device in FIG.13B to another device.

The storage media inserting portion 1026 is provided on the housing 1021a. The storage medium insertion portion 1026 may be provided on thehousing 1021 b. Alternatively, a plurality of storage medium insertionportions 1026 may be provided on one or both of the housings 1021 a and1021 b. For example, when a card-type recording medium is inserted intothe recording medium insertion portion, data can be read to theelectronic device from the card-type recording medium or data stored inthe electronic device can be written to the card-type recording medium.

The speaker 1027 is provided on the housing 1021 b. The speaker 1027outputs sound. Note that the speaker 1027 may be provided on the housing1021 a.

Note that the housing 1021 a or the housing 1021 b may be provided witha microphone, in which case the electronic device in FIG. 13B canfunction as a telephone set, for example.

In the electronic device illustrated in FIG. 13B, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1021 a or the housing 1021 b.

The electronic device illustrated in FIG. 13B functions as one or moreof a telephone set, an e-book reader, a personal computer, and a gamemachine, for example.

An electronic device illustrated in FIG. 13C is an example of astationary information terminal. The stationary information terminalillustrated in FIG. 13C includes a housing 1031, a panel 1032 providedon the housing 1031, a button 1033, and a speaker 1034.

The panel 1032 is a display panel (display). The panel 1032 preferablyhas a function of a touch panel.

Note that a panel similar to the panel 1032 may be provided on a deckportion 1035 of the housing 1031. This panel preferably has a functionof a touch panel.

The housing 1031 may be provided with one or more of a ticket slot fromwhich a ticket or the like is dispensed, a coin slot, and a bill slot.

The button 1033 is provided on the housing 1031. When the button 1033 isa power button, for example, pressing the button 1033 can control thesupply of a power voltage to the electronic device.

The speaker 1034 is provided on the housing 1031. The speaker 1034outputs sound.

In the electronic device illustrated in FIG. 13C, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1031.

The electronic device illustrated in FIG. 13C functions as, for example,an automated teller machine, an information communication terminal forordering a ticket or the like (also referred to as a multi-mediastation), or a game machine.

FIG. 13D illustrates another example of a stationary informationterminal. The electronic device illustrated in FIG. 13D includes ahousing 1041, a panel 1042 provided on the housing 1041, a support 1043supporting the housing 1041, a button 1044, a connection terminal 1045,and a speaker 1046.

Note that a connection terminal for connecting the housing 1041 to anexternal device may be provided.

The panel 1042 functions as a display panel (display).

The button 1044 is provided on the housing 1041. When the button 1044 isa power button, for example, pressing the button 1044 can control thesupply of a power voltage to the electronic device.

The connection terminal 1045 is provided on the housing 1041. Theconnection terminal 1045 is a terminal for connecting the electronicdevice in FIG. 13D to another device. For example, when the electronicdevice in FIG. 13D and a personal computer are connected with theconnection terminal 1045, an image corresponding to a data signal inputfrom the personal computer can be displayed on the panel 1042. Forexample, when the panel 1042 of the electronic device illustrated inFIG. 13D is larger than a panel of an electronic device connectedthereto, a displayed image of the electronic device can be enlarged, inwhich case a plurality of viewers can recognize the image at the sametime with ease.

The speaker 1046 is provided on the housing 1041. The speaker 1046outputs sound.

In the electronic device illustrated in FIG. 13D, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1041.

The electronic device illustrated in FIG. 13D functions as one or moreof an output monitor, a personal computer, and a television set, forexample.

FIG. 13E illustrates an example of an electric refrigerator-freezer. Theelectronic device illustrated in FIG. 13E includes a housing 1051, arefrigerator door 1052, and a freezer door 1053.

In the electronic device illustrated in FIG. 13E, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1051. With this structure, supply of a power voltageto the semiconductor device in the housing 1051 can be controlled inresponse to opening and closing of the refrigerator door 1052 and thefreezer door 1053, for example.

FIG. 13F illustrates an example of an air conditioner. The electronicdevice illustrated in FIG. 13F includes an indoor unit 1060 and anoutdoor unit 1064.

The indoor unit 1060 includes a housing 1061 and a ventilation duct1062.

In the electronic device illustrated in FIG. 13F, the semiconductordevice which is one embodiment of the present invention is providedinside the housing 1061. With this structure, supply of a power voltageto the semiconductor device in the housing 1061 can be controlled inresponse to a signal from a remote controller, for example.

Note that although the separated air conditioner including the indoorunit and the outdoor unit is shown in FIG. 13F as an example, it may bean air conditioner in which the functions of an indoor unit and anoutdoor unit are integrated in one housing.

The above is a description of the electronic devices illustrated inFIGS. 13A to 13F.

As has been described with reference to FIGS. 13A to 13F, the electronicdevices in this embodiment consume less power by using the semiconductordevice which is one embodiment of the present invention.

This application is based on Japanese Patent Application serial No.2012-193330 filed with Japan Patent Office on Sep. 3, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. (canceled)
 2. A semiconductor device comprising:a power source circuit comprising: a voltage converter circuitconfigured to generate a potential by using a first clock signal, thevoltage converter circuit comprising a first transistor; and a CPU corecomprising: a register comprising: a first memory circuit configured tostore data, the first memory circuit being supplied with a second clocksignal; and a second memory circuit configured to store the data outputfrom the first memory circuit, the second memory circuit comprising asecond transistor and a first capacitor, wherein a gate of the firsttransistor is electrically connected to one of a source and a drain ofthe first transistor, wherein a back gate of the first transistor iselectrically connected to the one of the source and the drain of thefirst transistor, wherein one of a source and a drain of the secondtransistor is electrically connected to the first capacitor, and whereina back gate of the second transistor is capable of being supplied withthe potential.
 3. The semiconductor device according to claim 2, whereineach of the first transistor and the second transistor comprises achannel formation region comprising an oxide semiconductor.
 4. Thesemiconductor device according to claim 2, wherein an output terminal ofthe voltage converter circuit is electrically connected to the one ofthe source and the drain of the first transistor.
 5. The semiconductordevice according to claim 4, wherein the voltage converter circuitfurther comprises a second capacitor, wherein a first electrode of thesecond capacitor is electrically connected to the other of the sourceand the drain of the first transistor, and wherein a second electrode ofthe second capacitor is supplied with the first clock signal.
 6. Thesemiconductor device according to claim 5, wherein the first electrodeof the second capacitor is electrically connected to the other of thesource and the drain of the first transistor via at least a thirdcapacitor.
 7. The semiconductor device according to claim 2, wherein anoutput terminal of the voltage converter circuit is electricallyconnected to the other of the source and the drain of the firsttransistor.
 8. The semiconductor device according to claim 7, whereinthe back gate of the first transistor is electrically connected to theone of the source and the drain of the first transistor via at least athird transistor.
 9. The semiconductor device according to claim 7,wherein the voltage converter circuit further comprises a secondcapacitor, wherein a first electrode of the second capacitor iselectrically connected to the one of the source and the drain of thefirst transistor, and wherein a second electrode of the second capacitoris supplied with the first clock signal.
 10. The semiconductor deviceaccording to claim 9, wherein the first electrode of the secondcapacitor is electrically connected to the one of the source and thedrain of the first transistor via at least a third capacitor.
 11. Asemiconductor device comprising: a power source circuit comprising: afirst voltage converter circuit configured to generate a first potentialby using a first clock signal, the first voltage converter circuitcomprising a first transistor; and a second voltage converter circuitconfigured to generate a second potential by using a second clocksignal, the second voltage converter circuit comprising a secondtransistor; and a CPU core comprising: a register comprising: a firstmemory circuit configured to store data, the first memory circuit beingsupplied with a third clock signal; and a second memory circuitconfigured to store the data output from the first memory circuit, thesecond memory circuit comprising a third transistor and a firstcapacitor, wherein a gate of the first transistor is electricallyconnected to one of a source and a drain of the first transistor,wherein a back gate of the first transistor is electrically connected tothe one of the source and the drain of the first transistor, wherein agate of the second transistor is electrically connected to one of asource and a drain of the second transistor, wherein a back gate of thesecond transistor is electrically connected to the one of the source andthe drain of the second transistor, wherein one of a source and a drainof the third transistor is electrically connected to the firstcapacitor, and wherein a back gate of the second transistor is capableof being supplied with the first potential or the second potential. 12.The semiconductor device according to claim 11, wherein each of thefirst transistor, the second transistor and the third transistorcomprises a channel formation region comprising an oxide semiconductor.13. The semiconductor device according to claim 11, wherein an outputterminal of the first voltage converter circuit is electricallyconnected to the one of the source and the drain of the firsttransistor, and wherein an output terminal of the second voltageconverter circuit is electrically connected to the other of the sourceand the drain of the second transistor.
 14. The semiconductor deviceaccording to claim 13, wherein the back gate of the second transistor iselectrically connected to the one of the source and the drain of thesecond transistor via at least a fourth transistor.
 15. Thesemiconductor device according to claim 13, wherein the first voltageconverter circuit further comprises a second capacitor, wherein thesecond voltage converter circuit further comprises a third capacitor,wherein a first electrode of the second capacitor is electricallyconnected to the other of the source and the drain of the firsttransistor, wherein a second electrode of the second capacitor issupplied with the first clock signal, wherein a first electrode of thethird capacitor is electrically connected to the one of the source andthe drain of the second transistor, and wherein a second electrode ofthe second capacitor is supplied with the second clock signal.
 16. Thesemiconductor device according to claim 15, wherein the first electrodeof the second capacitor is electrically connected to the other of thesource and the drain of the first transistor via at least a fourthcapacitor, and wherein the first electrode of the third capacitor iselectrically connected to the one of the source and the drain of thesecond transistor via at least a fifth capacitor.
 17. A semiconductordevice comprising: a power source circuit comprising: a first voltageconverter circuit configured to generate a first potential, the firstvoltage converter circuit comprising a first transistor; and a secondvoltage converter circuit configured to generate a second potential, thesecond voltage converter circuit comprising a second transistor; anoscillator configured to output a clock signal to the power sourcecircuit; and a CPU core configured to control whether an operation ofthe oscillator is stopped or not, the CPU core comprising: a registercomprising: a first memory circuit configured to store data in a periodduring which a power source voltage is applied to the CPU core; and asecond memory circuit configured to store data in a period during whichsupply of the power source voltage to the CPU core is stopped, thesecond memory circuit comprising a third transistor and a firstcapacitor, wherein a gate of the first transistor is electricallyconnected to one of a source and a drain of the first transistor,wherein a back gate of the first transistor is electrically connected tothe one of the source and the drain of the first transistor, wherein agate of the second transistor is electrically connected to one of asource and a drain of the second transistor, wherein a back gate of thesecond transistor is electrically connected to the one of the source andthe drain of the second transistor, wherein one of a source and a drainof the third transistor is electrically connected to the firstcapacitor, and wherein the CPU core is further configured to controlwhether the first potential or the second potential is applied to a backgate of the third transistor.
 18. The semiconductor device according toclaim 17, wherein each of the first transistor, the second transistorand the third transistor comprises a channel formation region comprisingan oxide semiconductor.
 19. The semiconductor device according to claim17, wherein an output terminal of the first voltage converter circuit iselectrically connected to the one of the source and the drain of thefirst transistor, and wherein an output terminal of the second voltageconverter circuit is electrically connected to the other of the sourceand the drain of the second transistor.
 20. The semiconductor deviceaccording to claim 19, wherein the back gate of the second transistor iselectrically connected to the one of the source and the drain of thesecond transistor via at least a fourth transistor.
 21. Thesemiconductor device according to claim 19, wherein the first voltageconverter circuit further comprises a second capacitor, wherein thesecond voltage converter circuit further comprises a third capacitor,wherein an electrode of the second capacitor is electrically connectedto the other of the source and the drain of the first transistor, andwherein an electrode of the third capacitor is electrically connected tothe one of the source and the drain of the second transistor.
 22. Thesemiconductor device according to claim 21, wherein the electrode of thesecond capacitor is electrically connected to the other of the sourceand the drain of the first transistor via at least a fourth capacitor,and wherein the electrode of the third capacitor is electricallyconnected to the one of the source and the drain of the secondtransistor via at least a fifth capacitor.